In conventional multiple-channel radio transmitters, a transmission signal is distributed over a broad transmission bandwidth, which is in turn divided into a plurality of sub-bands, referred to as “channels”. The channels are configured in a bank, each sub-band channel in the bank having a dedicated transmitter tuned exclusively to the sub-band of that specific channel. Each of the transmitters requires a significant degree of power, and each is bulky in size and is expensive to produce. This form of redundant architecture results in a large number of dedicated transmitters located at a base station. Not only is this type of dedicated channel architecture expensive, but each channel is also custom-built for a given air interface/modulation standard, and tuned for a given channel setting. It is therefore difficult or impossible to adapt a given channel for use in an environment having different transmission parameters.
Recent developments in digital signal processing (DSP) and data conversion have provided more efficient architectures radio transmitter designs. In the field of wireless base stations for example, wide-band transmitters have conferred significant benefits, including reductions in base station cost, size, complexity, and power consumption.
With reference to FIG. 1, in a wide-band transmitter 20, a plurality of DSP circuits 22 process the signals of a number of individual channels Ch1, Ch2, . . . ChL. Each channel transports information from a number of users. The channels are base-band filtered in order to band-limit the transmitted signal and to provide needed suppression outside the band of interest so as to avoid interference with adjacent channels. Base-band filtering can optionally be combined with an interpolation technique for improving the performance of a transmitter.
The digital base-band signals 23 are next input to a plurality of digital tuners 24, each tuner 24 filters and/or interpolates the digital base-band signal 23. The tuner mixes, or “up-converts”, the wide-band, interpolated signal to an intermediate frequency (IF), and a plurality of signals are combined. The wide-band digital signal 25 is next converted to an analog signal 27 by means of a digital-to-analog converter 26. The resulting analog signal 27 is band-shifted to a radio frequency (RF) at RF up-converter 28, and amplified by means of a power amplifier 30 and transmitted through an antenna 32.
While the conventional transmitter of FIG. 1 represents an improvement over prior techniques, its design is still limited in that it requires dedicated pre-programmed filters that are not readily modifiable to different channel parameters or modulation protocols or interface standards.
Therefore, in view of the limitations discussed above, it is desirable for a transmitter to be configured to operate as a conventional digital Finite Impulse Response (FIR) or Infinite Impulse Response (IIR) filter and to provide, through pulse-shaped filtering, pulse-shaped data samples at its output. It is further desired that a wide-band digital transmitter system be flexible so that it is simultaneously supportive of a variety of air interface/modulation techniques and protocols (e.g., AMPS, IS-136, GSM, EDGE, etc.), as well as being switchable between protocols whenever required.